Japanese Patent No. 2540738 shows a conventional system which utilizes a waste hest of an internal combustion engine. This system includes a pressure pump and a heater both of which are arranged in parallel to an evaporator of the refrigerant cycle. The pressure pump pressurizes a refrigerant in the refrigerant cycle, and the heater heats the refrigerant in the refrigerant cycle by performing heat-exchange between the engine coolant and the refrigerant. A three-way valve can be switched in such a manner that the pressure pump and the heater are selectively fluidly connected to the refrigerant cycle. A compressor driven by the engine serves as an expansion device, which expands a refrigerant to change a refrigerant pressure to a kinetic energy.
When the refrigerant flows through the pressure pump and the heater, a Rankine cycle is established using the compressor as the expansion device. The heat energy derived from the engine coolant is recovered as the kinetic energy. The kinetic energy is returned to the engine to improve the fuel economy.
The above Japanese patent shows control conditions of the each component such as the three-way valve, the pressure pump, and the compressor. However, is does not show the appropriate control to change the cycle between the refrigerant cycle and the Rankine cycle.
The inventors of the present invention experimentally confirmed following facts. In the case that the operation cycle is changed from the refrigerant cycle to the Rankine cycle, if the pressure pump is operated and the expansion device is in an operational condition, the refrigerant flows into the expansion devise and is expanded therein, so that the pressure pump has a difficulty to increase the pressure of the refrigerant in a short period whereby the energy is hardly recovered effectively. If the energy is not recovered in a short period, the air-conditioning may be deteriorated.
Furthermore, when the Rankine cycle is changed to the refrigerant cycle, since the lower-side pressure of the refrigerant at the starting of the compressor is higher than that at the ordinary starting of the compressor in the refrigerant cycle, a higher starting torque than usual is necessary to start the compressor. Thus, the power loss of the compressor is increased, so that the engine hardly starts smoothly. When the operation cycle is changed into the refrigerant cycle while the pressurized refrigerant by the pressure pump is not used as an effective power, the working of pressure pump is wasted and the power loss of the pressure pump is increased.
As described above, when the refrigerant cycle and the Rankine cycle are alternatively operated, various losses are generated and the energy efficiency is not well improved.